Horseshoe crabs are the supreme, if offbeat, survivors of the invertebrate world. Often called “living fossils” for visually obvious reasons – horseshoe crabs resemble a sort of prehistoric helmet (their Japanese name translates as “helmet crab”) – these animals have barely changed appearance along a lineage that stretches back 445m years. When situated dome-up, horseshoe crabs have a formidable defence against predators because of their shells. View image in fullscreen An adult mangrove horseshoe crab on a beach in Cox’s Bazar, Bangladesh. Female horseshoe crabs will deposit about 4,000 eggs in clusters in the sand, usually at high tide and during a full moon.